Collapse the scattered ensure_agent_runtime_dir calls into the lifecycle
functions themselves so callers have a single responsibility:
- lifecycle::spawn: calls ensure_agent_runtime_dir before write_dropins.
Callers (handle_spawn, ensure_root_agent) no longer need a separate
preamble step.
- lifecycle::rebuild_no_meta spawn path: calls ensure_agent_runtime_dir
before write_dropins. apply_commit / merge_config_pr flows no longer
need a manual ensure_agent_runtime_dir.
- run_create (job-queue): drops ensure_agent_runtime_dir + register_agent.
The tail Reconcile's converge_start_preamble handles the runtime dir
and mcp_sockets::spawn_poll handles the listener. Create stays purely
'provision + create', not 'create + start'.
- handle_spawn (server.rs): drops manual preamble; lifecycle::spawn owns it.
Drops unneeded unregister_agent on failure (supervisor handles listener).
- ensure_root_agent (auto_update.rs): drops manual ensure_agent_runtime_dir.
- actions.rs apply_commit / merge_config_pr: drop manual
ensure_agent_runtime_dir; rebuild_no_meta's spawn path handles it.
Result: ensure_agent_runtime_dir lives in exactly two places —
lifecycle::spawn (direct spawn) and converge_start_preamble (start/reconcile
path). All other callers are clean call sites.
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